MRI (Magnetic Resonance Imaging) and NMR (Nuclear Magnetic Resonance) systems use a large DC field to magnetise the sample or patient, plus smaller gradient fields and RF coils. To achieve the best imaging performance, the MRI system must be located in a stable magnetic field.

Presence of DC and AC fields external to the system will cause blurry images. This is why a site survey is essential in determining the optimum location for an MRI or NMR system. The optimum location has the lowest amount of interference, and where mitigation measures can be employed.

A site survey is carried out using a Spectramag-6, and a Mag-13 or Mag690 three-axis sensor. Magnetic field variations are recorded over time at the site. The period of time depends on the interference of interest, but for DC the longer the survey the better, in order to obtain a more representative view of daily activity (for example, around the site in a hospital). Sources of disturbance for DC and low frequencies include passing traffic and lifts; for AC the mains network and electromechanical equipment may contribute.

Accelerometers may be used to obtain vibration measurements. The equipment is also suitable for measurement of the 5 Gauss line, which marks the limit for field exposure to the general public (external link).